What's the cost difference for Motronic tuning an EFR vs. billet k26, on a UrS4?

depends. for those turbos that fall within the range of the stock MAF (the 6758 on pump gas will be) its the same price. ($500 without launch control, $550 with launch control. there will also be package discounts with purchase of turbo available)

for the higher power stuff we need speed density or big maf (rs4) which I charge $250-$350 more for depending on configuration.